The exposure of powdered unconfined coals to CO2 results in changes in the coals' physical structures. The presence of water changes the behavior of the coals on exposure to CO2. The sorption of CO2 on seven Argonne premium coals was measured by using attenuated total reflectance- Fourier transform infrared (ATR-FTIR) spectroscopy as a function of time at constant CO2 pressure (similar to 0.62 MPa) and temperature (55 degrees C). The depth sampled is at least 1 mu m, and this ensures that both the bulk and surface of the sample were interrogated. Concentrations of CO2 in the top 1-7 Am of the coal were measured by using the peak area of the CO2 ATR-FTIR band near 2333 cm(-1). Diffusion rates were measured by using the time dependence of the area of the 2333 cm(-1) band. Surface adsorption is effectively instantaneous. The coals were either extensively or briefly dried. The coals were exposed to CO2, evacuated, and then exposed to CO2 a second time. For the extensively dried coals, removal of the CO2 under vacuum was much faster than CO2 sorption, indicating a coal structure change caused by CO2 sorption. The diffusion rate of the CO2 into the coal was much faster for the second exposure, confirming that the coal underwent a physical structure rearrangement. Structure rearrangement was observed for all seven extensively dried coals and for the Pittsburgh No. 8 briefly dried coal. The presence of residual moisture in the briefly dried coal samples appeared to inhibit or block CO2 uptake as equilibrium was reached within minutes. The second exposure of the briefly dried coals universally resulted in greater CO2 uptake that was again instantaneous on our experimental time scale.